1. Field of the Invention
This invention relates generally to analytical optical instruments, particularly spectrophotometers, and more specifically to spectrophotometers which incorporate a monochromator for producing light in a range of wavelengths.
2. Description of the Prior Art
The analytical function of spectrophotometric instruments is achieved by measuring light of selected wavelengths absorbed or emitted by an unknown sample composition. Conventionally, light from a spectral source is passed through a monochromator which contains a dispersive element such as a diffraction grating. Light entering the monochromator strikes the grating and is dispersed to form a light spectrum which is optically conducted to impinge on a narrow exit slit which limits the light issuing from the monochromator at a given instant to a very narrow band of wavelengths. The dispersion element is angularly displaceable to vary the portion of the spectrum impinging on the slit and thereby produces what is known as a "wavelength scan".
In a double-beam null type instrument to which the present invention is addressed, light emanating from the monochromator is split into a sample beam and a reference beam; the sample beam is transmitted through a cell containing the unknown sample composition whereas the reference beam bypasses the sample. A servo motor-controlled optical attenuator member is adjustably disposed in the path of the reference beam. The respective intensities of the sample and reference beams are sensed by suitable detectors and the intensity of the sample beam compared with that of the reference beam to generate an error signal fed back to the servo motor drive which moves the attenuator so as to achieve a null condition. The position of the attenuator therefore is representative of the intensity of the sample beam and, in a typical instrument, the attenuator is coupled to effect ordinate displacement of the stylus of a strip chart recorder, the abscissa of the chart corresponding to the instaneous wavelength of radiation from the monochromator.
It is well known that the intensity of the light issuing from the monochromator, even over the relatively narrow band of frequencies constituting the spectral line for a given angular position of the dispersion element, has a generally Gaussian distribution. Consequently, if, as is the case in prior art instruments, the wavelength scan is accomplished at a uniform rate, it is not possible to adjust the position of the attenuator as rapidly as the changes of intensity occur during certain portions of the scan thereby causing the position of the attenuator to be an inexact representation of the detector sample beam.